Preparation and Characterization of CdS Thin Films

H. Moualkia 1,*, N. Attaf 2, L. Hadjeris 1, L. Herissi 1, N. Abdelmalek1

1 Laboratoire des matériaux et structure des systèmes électromécaniques et leur Fiabilité, Faculté

des sciences et de la technologie, Université Larbi Ben M’Hidi de Oum El Bouaghi, Algérie 2 Laboratoire de couches minces et interfaces, Faculté des sciences, Université Mentouri de

Constantine, Algérie

Corresponding author: moualkia88@ yahoo.fr

ABSTRACT In this work we report the structural, optical and electrical properties of CdS thin films

prepared by chemical bath deposition (CBD) under the effect of solution temperatures. The

solution temperatures used vary between 55 and 75 °C. The XRD patterns show that the films

have a hexagonal phase with a preferential (002) orientation. The structural parameters such as

the grain size, dislocation density and strain are calculated. The transmission spectra, recorded in

the UV visible range, reveal a high transmission coefficient (85%) of the prepared films and an

optical band gap values of 2 - 2.4 eV. The electrical measurements show that the dark

conductivity values increase from 10-7 to 10-4 (Ω.cm)-1 at higher temperature (Ts > 65°C). It is

found that the photoconductivity of the deposited films is two to five decades larger than the dark

conductivity, and the photoconductivity to the dark conductivity ratio obtained at 3000 Lx vary

from 102 to 105. From these results we inferred that the elaborated CdS thin films exhibit good

properties intended for solar cell window layers.

Index Terms-CdS, Thin films, Chemical bath deposition (CBD), Photoconductivity, Solar cell

1. INTRODUCTION

Solar Energy is one of the abundant,

non polluting renewable energy of our planet.

During the last three decades considerable

progress has been made in developing

technologies to harness electricity from solar

radiation. The most commonly used solar cell

material is crystalline silicon (Si) and

naturally the cost is an obstacle to terrestrial

applications. The development of thin film

solar cells is an active area of research at this

time. Recently, much attention has been paid

to the development of low cost, high

efficiency thin film solar cells. Cadmium

2012 First International Conference on Renewable Energies and Vehicular Technology

978-1-4673-1170-0/12/$31.00 ©2012 IEEE 66

sulphide (CdS) is one of the low cost

materials of this kind. CdS is known to be an

excellent heterojunction partner for p-type

cadmium telluride (CdTe) and p type copper

indium diselenide (CuInSe2) due to its wide

band gap, high absorption coefficient and

photoconductivity. It has been widely used as

a window material in high efficiency thin film

solar cells based on CdTe or CIGS [1, 2].

Moreover, the latter application needs CdS

thin film with (i) high structural orientation,

(ii) high optical transmittance to allow the

sunlight to enter the absorbing material more

readily, (iii) relatively large conductivity to

reduce the electrical solar cells losses and

higher photoconductivity that causes a smaller

series resistance in the cells.

Several methods were used to prepare CdS

thin films such as electrodeposition, spray,

sputtering and chemical bath deposition

(CBD). Among these methods chemical bath

deposition is well investigated and widely

employed to obtain CdS films for

photovoltaic applications [4]. Indeed, J. N.

Ximello-Quiebras [4] has investigated the

physical properties of chemical bath deposited

CdS thin films. J.Y. Choi [5], using different

chemical reagents and ultrasonic agitation,

achieves hexagonal CdS films oriented in the

(101) direction. C. Guillén et al. [6] reported

that the CdS films prepared by CBD method

using low cadmium salt concentration and a

high thiourea one in the bath, exhibited a high

transparent CdS films. U. S. Jadhav at al. [7]

have studied the effect of Cd:S ratio on

photoconductivity of CdS films. This research

work aims at studying the influence of

solution temperature on the structural, optical

and electrical properties of CdS thin films

deposited by CBD. The samples are

illuminated at different light intensity of 220,

630 and 3000 Lx in order to understand the

influence of the latter parameter on the

photoconductivity of the elaborated films.

2. EXPERIMENTAL DETAILS

The CdS thin films are deposited onto

glass substrates by chemical bath deposition

technique. The used solution is prepared in

100 ml beaker by the sequential addition of

distilled water, ammonia NH4OH (2M)

cadmium acetate Cd(CH3COO)22H2O (5.10-3

M), Thiourea SC(NH2)2 (2.10-2 M). A total

volume of 78.5 ml of the chemical bath was

formed after mixing the different components.

Before deposition, the glass substrates are

cleaned in acetone and methanol

ultrasonically, rinsed in distilled water and

dried in hot air. Then, the glass substrates are

vertically immersed into the chemical bath

solution. The solution temperatures are vary

from 55 to 75°C. The deposition time is 60

min for each film. After deposition, the film

is retired from the chemical bath, cleaned with

distilled water and dried in air at room

temperature.

The films’ thicknesses are measured using a

profilometer DEKTAK 3ST. The structural

67

characterization of the films is carried out by

the X-ray diffraction (XRD) technique using

an X-ray diffractometer (Philips X’Pert) with

αKCu radiation. The optical transmittance of

the films is studied using a Shimadzu 3101

PC UV-visible spectrophotometer. The optical

gap is deduced from the recorded

transmittance spectra. The electrical

conductivity and the photoconductivity of the

films are measured in a coplanar structure

obtained by evaporation of two golden strips

on the film surface. For the photoconductivity

measurements, the samples are illuminated by

unfiltered white light from a halogen lamp

whose light intensities are 220, 630 and 3000

Lx

3. RESULTS AND DISCUSSION

3.1 Structural properties

Figure 1 shows the XRD patterns of the

samples deposited at different temperatures.

The diffraction patterns show a diffraction

peak located at 26. 75° corresponding

respectively to (002) plane of hexagonal CdS

according to the JCPDS data (6-314) [8]. The

same structure in the CdS thin films deposited

by CBD has been reported by earlier reports

[9-12]. Note that for solar cell applications,

hexagonal CdS thin films are preferable due

to their excellent stability [13].

From the DRX pattern, it is clear that the

(002) diffraction peak disappears at 75°C.

This is due probably to the defect formation

such as sulphur vacancies Vs and cadmium

interstices Icd at higher temperature as can be

seen in more detail below. It is well

established that the defects formation reduces

the crystalinity [14, 15, 16, 17].

The grain size (D), dislocation density (δ) and

the strain (ε) are calculated using respectively

the relations 1, 2 and 3 given below and

reported in table 1:

θβ

λcos9.0=D (1)

21

D=δ (2)

4cosθβε = (3)

Where, λ is the wavelength of the incident X-

rays, θ is the incidence angle and β is the full

width at half maximum (FWHM) of the

diffraction peak.

Approximately similar structural parameters

values were reported in the literature [9, 18].

Table 1

Structural parameters of the CdS films (Grain

size (D), dislocation density ( )δ , strain ( )ε )

Sample

temperature

Ts (°C)

D (nm)

δ (×1015

(lines/m2))

ε (10-3)

55 20,93 2,282 1,654

60 17,56 3,243 1,972

65 18,35 2,969 1,888

75 - - -

68

3.2 Optical properties

Figure 2 shows the optical transmittance of

CdS films deposited at various solution

temperatures. It is clear from the figure that

the transmittance increases sharply in the

range 400-450 nm. Then all films exhibit

optical transmittance more than 60 % above

500 nm, which is one of the prerequisites for

solar cells window layer [19]. We note that

the transmittance is extended to lower

wavelength up to 300 nm. This is due

probably to the disorder effects or to the

presence of the amorphous components in the

film [4]. The absorption coefficient α of CdS

thin films is calculated from the transmittance

spectra using the Beer-Lambert

approximation.

200 400 600 800 1000 1200-10

0

10

20

30

40

50

60

70

80

90

Tran

smitt

ance

(%)

Ts = 55°C Ts = 60°C Ts = 65°C Ts = 75°C

Figure 2: Influence of solution temperature

on the optical transmittance of CdS films

20 30 40

10

20

30

40

(002)H, ( 2θ = 26,75° )

Ts= 55°CIn

tens

ity I

(u.a

)

2θ (°)

20 30 40

10

20

30

40

Ts= 65°C (002)H, ( 2θ = 26,75° )

Inte

nsity

I(u.

a)

2θ (°)20 25 30 35 40

10

20

Ts= 75°C

Inte

nsity

I(u.

a)

2θ (°)

20 25 30 35 40 450

10

20

30

40

50

60

70

80

(002)H, ( 2θ = 26,75° )

Inte

nsity

I(u.

a)

2θ (°)

Ts=60°C

Figure 1: XRD spectrum of CdS thin films under different solution temperatures

69

The absorption coefficient α can be expressed

by the standard expression for direct

transition,

υυ

αh

EghA −= (4)

Where A is a constant, Eg is the energy band

gap, υ is the frequency of the incident

radiation and h is Plank’s constant. The

energy gaps of the films have been

determined by extrapolating the linear portion

of the plots of (αhυ)2 against hυ to the

energy axis. The obtained values are in the

range of (Eg = 2 - 2.4 eV) (see table 2) in

agreement with earlier findings [18, 20].

3.3 Electrical properties

The conductivity is measured in the dark

at different temperatures. The variation of the

dark conductivity and its activation energy EA

as a function of the solution temperature are

presented in figure 3. The EA values are

deduced from the variation of the conductivity

versus temperature as shown in our previous

work [21].

The electrical activation energy EA=

EC-EF, indicates the Fermi level position EF

regarding the minimum of the conduction

band EC and thereafter the free carriers

concentration variation. To determine the

Fermi level position in the forbidden band,

one calculates the ratio (2EA/Eg). The results

are presented in table 2.

As can be seen from table 2, the (2EA/Eg)

values are less than the unity for all solution

temperatures indicating that the deposited

films are of n-type. This is in agreement with

earlier findings [13, 22].

Table 2: Optical bandgap (Eg) and (2EA/Eg) ratio

of CBD-CdS films.

As shown in figure 3, for solution

temperature Ts ≤ 65°C, the dark conductivity

varies from 10-6 to 10-7 (Ω.cm)-1. This low

dark conductivity may be interpreted by a

decrease of the carrier concentration since the

electrical conductivity depends on the number

of charge carriers. It may also be due to the

presence of structural disorders and

dislocations as interpreted by D. Padiyan and

al [23].

Sample

temperature (°C)

Eg (eV)

EA

(2EA/Eg)

55 2,3 0.24 0.208 60 2,2 0.32 0.290 65 2,4 0.40 0.333 75 2 0.13 0.13

70

In the high temperature region (Ts >

65°C), the dark conductivity increases from

10-7 to 10-4 (Ω.cm)-1. This increase of the dark

conductivity is due principally to the sulphur

deficiency which is due either to the presence

of hydroxide cadmium Cd(OH)2 [24], since

the dominating growth mechanism is cluster

by cluster at higher temperature [25], and to

the sulphur volatility.

Consequently, we suggest that CdS films

deposited at high temperatures contain a high

concentration of sulphur vacancies Vs and

cadmium interstices Icd which act as donor’s

defects in CdS films.

3.4 Photoconductivity properties

The dark conductivity (σdark) and the

photoconductivity (σphot) as function of

solution temperature at different light

intensities 220, 630 and 3000 Lx are reported

in figure 4. The photoconductivity of the films

increases with increasing light intensity.

It is also seen from figure 4 that the

photoconductivity of the deposited films is

two to five orders of decade larger than the

dark conductivity. The photoconductivity to

the dark conductivity ratio obtained at 3000

Lx varies from 102 to 105. Similar results are

reported by C. Guillen et al. [6] in the same

temperature range.

The photoconductivity evolution as a

function of light intensity and temperature can

be interpreted as fellow:

- As the films are prepared by the chemical

bath deposition method, some oxygen can be

55 60 65 70 75

1E-6

1E-5

1E-4

1E-3

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

Dar

k co

nduc

tivity

(Ohm

.cm

)-1

Solution temperature Ts (°C)

Dark conductivity

Act

ivat

ion

ener

gy (e

V)

Activation energy

Figure 3: Dependence of dark

conductivity and electrical activation

energy on the solution temperature.

55 60 65 70 75

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

σ da

rk (O

hm.c

m)-1

σ ph

ot (O

hm.c

m)-1

Solution temperature Ts(°C)

Dark conductivity (σ dark) σ phot at 220 Lx σ phot at 630 Lx σ phot at 3000 Lx

Figure 4: Variation of the

photoconductivity σphot at different light

intensities 220, 630 and 3000 Lx as

function of solution temperature.

71

absorbed onto the surface and/or in the grain

boundaries of the CdS films [9, 26]. The

absorbed oxygen acting as an acceptor

impurity and as a trap for carriers. This

explains the low photoconductivity at 220 lx.

- At higher light intensities (630 and 3000 lx),

the oxygen is desorbed from the samples,

resulting in the rise of the photoconductivity

[6, 27]

- The low photoconductivity in the film

deposited at Ts = 60 °C is due to the

recombination velocity of the photocarriers.

At this solution temperature, the obtained

films present relatively high structural defects

as reported in table 1.

4. CONCLUSION

In this study, the CdS thin films were grown

by chemical bath technique. Structural,

optical, and electrical properties of the CdS

thin films have been investigated as a function

of solution temperature in order to optimize

their optoelectronic properties and prepare

promising films for window layer applications

in photovoltaic solar cells. The structural

studies revealed that the films have

preferential orientation along the (002) plane

of the hexagonal phase with good crystallinity

and grain size of about 17 nm. The optical

investigations showed that the CdS films have

good band gap in the range of 2-2.4 eV and

high optical transmission (85%) in the visible

range. It is found that solution temperature

had obvious effect on the dark conductivity.

The increase of solution temperature beyond

65°C improves the dark conductivity of the

CdS thin films. The photoconductivity was

found to increase with increasing light

intensity. These results indicate that the

elaborated CdS thin films exhibit good

properties intended for solar cell window

layers.

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